The invention relates to a circuit arrangement for protecting a transmission circuit - over which, for example, DC signals are transmitted and interpreted by means of threshold logic elements - from over-current damage by means of a line switch.
One problem when transmitting DC signals is that when faults occur, the current in the loop of the transmission line can be many times the customary operating current. However, since many sensitive circuit elements, e.g., transmitters, receivers, weighting networks, etc. are in series with the transmission circuit, over-current situations must be detected, and the current cut-off, as quickly as possible. Previously, protective elements were connected in series to the transmission circuits. For example, PTC thermistors were used which heat up when the current increases and whose resistance rises because of the higher temperature, or electronic fuses, e.g., simple transistor circuits were incorporated. The drawbacks of such circuit elements are that during normal use there occurs an additional voltage drop thereacross; further, the automatic resetting of an electronic fuse can only take place with great difficulty when used in a trunk line.